1. Field of the Invention
The present invention is directed to a magnetic resonance spectroscopy method.
2. Description of the Prior Art
Magnetic resonance spectroscopy has been employed for more than four decades in physical, chemical and biochemical basic research, for example as analysis technique or for obtaining structure information for complex molecules. Magnetic resonance spectroscopy, like magnetic resonance tomography, is based on the principle of magnetic nuclear resonance. The primary objective of spectroscopy, however, is not imaging but an analysis of a substance. Resonant frequencies of isotopes that have a magnetic moment, for example 1H, 13C or 31P, are thereby dependent on the chemical structure the molecules wherein the isotopes are bonded. A determination of the resonant frequencies therefore allows a differentiation to be made between different substances. The signal intensity at the various resonant frequencies provides information about the concentration of the corresponding molecules.
When a molecule is introduced into a basic magnetic field of a magnetic resonance apparatus, as occurs in the case of spectroscopy, electrons of the molecule shield the basic magnetic field of atomic nuclei of the molecule. As a result of this effect, the local magnetic field changes at the location of an atomic nucleus by a few millionths of the external basic magnetic field. The variation of the resonant frequency of this atomic nucleus connected therewith is referred to as chemical shift. Molecules thus can be identified on the basis of their chemical shift. Since frequency differences are easier to acquire by measurements, and can be acquired more precisely, than absolute frequencies, the chemical shift is indicated relative to a reference signal, for example the operating frequency of the magnetic resonance apparatus, in ppm.
A resonance line of an atomic nucleus can be split into a number of lines when further atomic nuclei having a magnetic moment are located in the environment of the atomic nucleus under consideration. The reason for this is due to phenomenon referred to as spin-spin coupling between the atomic nuclei. The magnetic flux density of the basic magnetic field that an atomic nucleus experiences thus is not only dependent on the electronic shield around this atomic nucleus but is also dependent on the orientation of the magnetic fields of the neighboring atoms.
Clinical magnetic resonance spectroscopy means magnetic resonance spectroscopy upon employment of clinical magnetic resonance apparatus. Methods for localized magnetic resonance spectroscopy essentially differ from those of magnetic resonance imaging in that the chemical shift is resolved in the spectroscopy in addition to the tomographic, topical resolution. Currently, two localization methods of magnetic resonance spectroscopy are dominant for clinical application. These are individual volume techniques based on echo methods wherein a spectrum of a previously selected target volume is registered, and spectroscopic imaging methods, referred to as CSI methods (chemical shift imaging) that simultaneously enable the registration of spectra of many spatially interconnected target volumes.
Spectroscopic imaging methods are employed in clinical phosphorous spectroscopy as well as in proton spectroscopy. A three-dimensional CSI method can include the steps of following a non-slice-selected 90° RF pulse, activating a combination of magnetic phase-coding gradients of the three spatial directions for a defined time, and subsequently reading out the magnetic resonance signal in the absence of any and all gradients. This is repeated with different combinations of phase-coding gradients until the desired topical resolution has been achieved. A four-dimensional Fourier transformation of the magnetic resonance signals supplies the desired, spatial distribution of the resonance lines. A two-dimensional CSI method arises from the three-dimensional method set forth above by replacing the non-slice-selected RF pulse by a slice-selective excitation composed of slice-selected RF pulse and corresponding magnetic gradient and one phase-coding direction is eliminated.
The single-volume techniques that are usually applied are based on an acquisition of a stimulated echo or of a secondary spin echo. In both instances, a topical resolution ensues by successive selective excitations of three orthogonal slices. A target volume is thereby defined by a section volume of these three slices. Only a magnetization of the target volume interacts with all three selective RF pulses and thus contributes to the stimulated echo or secondary spin echo. The spectrum of the target volume is obtained by one-dimensional Fourier transformation of a time signal corresponding to the stimulated echo or to the secondary spin echo. Compared to the method based on a secondary spin echo, only one-half of a magnetization of the target volume is rephased in the method based on a stimulated echo. In order to obtain short echo times for the stimulated echo or secondary spin echo, the power capability of a RF system of the magnetic resonance apparatus is essentially relevant given the method based on a secondary spin echo, in contrast to which the power capability of a gradient system of the apparatus is additionally relevant in the method based on the stimulated echo. Significantly lower gradient intensities are thereby required given the method based on a secondary spin echo. Since, given the method based on a stimulated echo, gradients having comparatively high intensity must be activated for a short echo time even immediately before an acquisition of the stimulated echo, there is a high susceptibility to disturbances caused by eddy currents.
In the single-volume technique based on a secondary spin echo, the following signals can be fundamentally observed after the free RF pulses: a three induction decay of each RF pulse, a spin echo for each RF pulse pair, as well as the secondary spin echo and a stimulated echo from all three RF pulses together. Only the secondary spin echo is to be measured, and all other signals are blanked out. This can ensue by activating additional magnetic gradients are such that the magnetization from the target volume is completely rephased at the point in time of the acquisition of the secondary spin echo. By contrast, the magnetization of the remaining volume is completely dephased. Gradients referred to as spoiler gradients are utilized for this purpose. The spoiler gradients can be activated as separate gradient pulses or can be realized by a lengthened activation duration of slice selection gradients over a transmission duration of an appertaining RF pulse, as described, for example, in U.S. Pat. No. 4,480,228. Regarding the blanking out of unwanted signals, further, the article by J. Hennig “The Application of Phase Rottion for Localized in Vivo Proton Spectroscopy with Short Echo Times”, Journal of Magnetic Resonance 96 (1992), pages 40 through 49, teaches that the phases of the three RF pulses be modified per repetition given a repeated registration of the secondary spin echo in conformity with a phase cycle. It is pointed out in the article that only the slice selection gradients in the two refocusing RF pulses are lengthened by 2 ms toward the leading back and trailing edges for spoiling in order to achieve short echo times. Other spoiler gradients are not activated.
Further, short echo times can be achieved with the ISIS method (image selected in vivo spectroscopy). The ISIS method is based on a sequence of eight non-selected 90° RF pulses with intervening, slice-selective 180° HF pulses. A 180° RF pulse for selecting slices is utilized three times, two 180° RF pulses for selecting line elements are utilized twice and three 180° RF pulses for selecting a target volume element are utilized once between two of the 90° RF pulses. Magnetic resonance signals are acquired following each 90° RF pulse, these magnetic resonance signals being subtracted or added for forming a signal that relates exclusively to the target volume element. One particular difficulty in the ISIS method results from the fact that—for an accurate determination of signal parts that relate exclusively to the target volume element—signal components of the other, generally much larger, overall volume must cancel exactly in the subtractions and additions. As a result, the ISIS method is very susceptible to even the minutest movements of the examination volume. Further details regarding the ISIS method are described, for example, in the article by R. J. Ordidge et al. “Image-Selected in Vivo Spectroscopy (ISIS). A New Technique for Spatially Selective NMR Spectroscopy”, Journal of Magnetic Resonance 66 (1986), pages 283 through 294.
The intense water signals are often suppressed in clinical proton spectroscopy. One method for such water suppression is, for example, the CHESS technique, whereby the nuclear spins of the water molecules are first selectively excited by narrow-band 90° RF pulses and their cross-magnetization is subsequently dephased by activating magnetic field gradients. In the ideal case, no detectible magnetization of the water molecules thus is available for an immediately following spectroscopy method.